A switching pre-regulator for a bulk capacitor filter followed by a series pass regulator has a switching element controller that relies upon a large desirable leakage inductance in a main secondary winding of a power transformer acting as a swinging choke input to the bulk capacitive filter. This desirable leakage inductance limits inrush current and supplies some filtering. However, the effective value of the swinging choke is a function of load conditions, and introduces a varying phase shift that would potentially disturb the zero crossing detection used in properly activating the switching element. In addition to creating a reference ramp synchronized to the phase shifted, twice power line frequency, full wave rectified zero crossings of the principal current supplying secondary winding, the switching element controller receives a selectable one of an isolated indication of primary line voltage zero crossings or zero crossing indications for an un-rectified auxiliary secondary winding that has a separate leakage inductance that mimics the effect of the phase shift experienced with the main secondary winding. This gives the switching controller the ability to know true zero crossings. The controller for the switching element can now adjust the conduction of that switching element to turn off at a zero crossing, which reduces both acoustic noise from the power transformer and in-circuit electrical noise by eliminating a steep falling edge in the switched current pulse used to supply the bulk capacitive filter.